ABSTRACT: Genetic Significance of Textural and Compositional Variations in Hydrothermal Manganese Oxide Deposits of the Mariana Island Arc
Marjorie S. Schulz, James R. Hein
Hydrothermal manganese oxides were recovered in 19 dredges along 1200 km of the active Mariana Island Arc. Two types of deposits occur, (1) Mn-oxide cemented volcaniclastic sediment, and (2) stratabound Mn oxides. Petrographic investigations of polished sections show a variety of textures: colloform, homogeneous and dense, laminated and dense, porous dendritic colloform, and scalloped. Generally, the textures have an internal framework of aligned Mn-oxide crystallites or isotropic Mn oxides, which are commonly intercalated. For example, colloform botryoids are composed of alternating laminae of aligned Mn-oxide crystallites and isotropic Mn oxides.
SEM-EDAX investigation of the polished sections using back-scattered electrons shows that textural differences reflect compositional differences. The laminae of aligned crystallites consistently have a higher mean atomic number, as indicated by higher back-scatter, than the isotropic Mn-oxide laminae. Spectral analysis (EDX) of the aligned crystallites and isotropic Mn-oxide laminae from the same colloform botryoid showed no spectral (elemental) differences for the elements above the atomic number of neon. The compositional difference indicated by backscattered electrons is most likely due to different water and/or oxygen contents with the laminae of aligned crystallites containing less structural water, less absorbed water, or less oxygen. One possible explanation is that areas of al gned crystallites formed under conditions that were more reducing than adjacent isotropic laminae and therefore contain less oxidized manganese. These deposits could form when reduced hydrothermal fluids variably mix with oxygenated pore waters within the sediment pile.
An alternative explanation of the compositional variation is differing water content of the Mn-oxide laminae. Isotropic Mn-oxide laminae with smaller crystallite size has greater surface area, thus greater water content. In either explanation, compositional changes within individual colloform or layered sequences probably reflect waxing and waning hydrothermal events.
AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990